This invention relates to a method and an apparatus for continuously annealing and pickling steel strips, particularly stainless steel strips, and more particularly to a method and an apparatus for continuously annealing and pickling stainless steel strips to realize advantageous improvement of annealing and pickling treatment faculty and effective shortening of treatment line.
Cold rolled stainless steel strips are generally subjected to the continuous annealing and pickling treatment in an annealing and pickling line which is referred to herein as "APL".
FIG. 1 schematically illustrates an APL as one example of prior art. It comprises a pay-off reel 11, a shearing machine 12 on an entry side, a welder 13, a looper 14 on the entry side, an annealing furnace 15 consisting of a heating portion 16 including a preheating, heating and soaking zones, and a cooling zone 17. This line further comprises a first neutral salt electrolytic cell 18, a second neutral salt electrolytic cell 19, a final treating bath 20, a scrubber 21, a drier 22, a looper 23 on an exit side, a shearing machine 24, and a tension reel 25.
In the above APL, a steel strip S unwounded from the pay-off reel 11 is cut at its leading end or trailing end with the shearing machine 12 at the entry side and is welded by the welder 13 to another preceding or following steel strip. The steel strip S is then introduced through the looper 14 into the annealing furnace 15 to be subjected to a predetermined heat-treatment. During this treatment, the steel strip is supported in catenary by asbestos rolls 16r and is subjected to heat-treatment by direct fire burners and then cooled in the cooling zone with the aid of water and/or air. Thereafter, the steel strip is subjected to descaling and a process for making it into passive state in the first and second neutral salt electrolytic cells 18 and 19 and the final treating bath 20. Na.sub.2 SO.sub.4 is used as the neutral salt. In the final treating bath 20, the pickling is effected by the use of HNO.sub.3 for ferrite stainless steel and a mixed acid of HNO.sub.3 and HF for austenite stainless steel. Such pickling procedures are summarized in FIG. 2.
After cleaning surfaces of the steel strip by the scrubber 21 and drying the steel strip by the drier 22, the steel strip passes through the looper 23 on the exit side and is cut by the shearing machine 24 on the exit side into predetermined lengths after which they are wound about the tension reel 25.
As shown in FIG. 1, horizontal furnaces supporting therein steel strips in catenary and equipped with direct fire burners are generally used for APL. The furnaces of this type are usually employed for the following reasons.
Annealing temperatures for ferrite stainless steels such as SUS 430 (Japanese Industrial Standard) are 780.degree.-850.degree. C. which are only somewhat higher than those of normal steel strips. However, annealing temperatures for austenite stainless steels such as SUS 304 (Japanese Industrial Standard) are 1,010.degree.-1,150.degree. C. which is very high. Owing to such a high temperature annealing, direct fire heating system has been used as heating means in consideration of productivity and preservation without using direct heating type heaters used for normal steels.
With the direct fire heating system, however, oxide films occur on surfaces of steel strips, so that descaling by pickling after annealing is absolutely necessary.
The oxide films produced on stainless steels are much denser and stronger than those of normal steels. Such oxide films become denser as the concentration of O.sub.2 in the furnace becomes to zero. In order to facilitate the descaling treatment in later pickling process, therefore, oxidizing atmosphere has been maintained in furnaces, whose oxygen concentration is of the order of 2-3%.
As a result, the oxidizing atmosphere increases oxide scales which tend to attach to hearth rolls and grow further to cause so-called "pick-up" defects. In order to prevent the "pickup" defects, the hearth rolls are made of asbestos and the number of the hearth rolls is made as small as possible to support the stainless steel strip in catenary (Japanese Patent Application Publication No. 26,723/77). As above described, the horizontal furnaces supporting therein steel strips in catenary and equipped with direct fire burners are mainly used for APL.
As above described, the continuous annealing and pickling lines (APL) of the prior art cannot avoid oxide scales which, however, are removed in later pickling treatment. This pickling treatment serves not only to descale but also to bring the steel strip to the passive state in order to improve its corrosion-resistance. For these purposes, nitric acid, sulfuric acid and mixed acid of nitric acid and hydrofluoric acid have been used in combination. Recently, salt bath, neutral salt electrolytic cell and the like have been used.
In the continuous annealing and pickling installations of the prior art as above described, the treating lines become greatly large and long, while production capacity still stays at the lower level.
In order to obtain a production capacity of 15 ton/h, for example, the horizontal furnace supporting steel strips in catenary and equipped with direct fire burners has a length of about 45 m and the pickling bath has a length of about 50 m, that is to say, the installation of overall length of as much as 100 m is needed.
In order to avoid the pickup defects, moreover, the asbestos rolls must be frequently exchanged such as a few times a month, so that there is a difficulty in productivity and maintenance.
Moreover, in addition to a plurality of pickling baths respectively including different pickling liquids, additional apparatuses such as acid solution supply apparatuses, acid solution circulating apparatuses. Furthermore, corrosion by the acid solutions makes difficult the maintenance. Moreover, these pickling solutions are troublesome in management.
Furthermore, an acid fume treating apparatus, a waste acid treating apparatus, a water treating apparatus and the like are needed for environmental sanitation.